In order to obtain a comprehensive idea of the climate of a continent, it is obviously desirable to subdivide it into areas having more or less similar and distinct atmospheric conditions. The leading difficulty in making such subdivisions is the well-known fact that the climate of any region which may be chosen passes by insensible gradations into that of adjacent regions, and any boundaries that may be drawn are to a considerable extent artificial and arbitrary. While the true basis on which to establish climatic areas or provinces is the resultant of all the weather elements which go to make up the atmospheric conditions recognised under the broader term climate, so many factors have to
be considered that it is extremely difficult to arrive at a general acceptable conclusion. The principal elements of the weather to be considered are, approximately, in the order of their importance, beginning with the one which exerts almost universal control—temperature, precipitation, the winds, absolute and relative humidity of the air, and evaporation.
Plate III.—Climate & life provinces. Click image to expand.
In North America there are voluminous records in reference to each of these elements of the weather, embracing considerable lengths of time, and relating mainly to the United States and southeastern Canada, but not sufficient to enable one to subdivide the entire continent into climatic provinces. The lack of weather records embracing the entire continent may be supplied in part by what may be termed the natural records of atmospheric conditions as expressed by the flora and fauna, as well as by soil conditions and topography. In recognition of this principle, the climatic provinces of North America here adopted have been made to conform to its life-zones.
The major climatic divisions of North America here provisionally adopted are, in their general order from south to north, the Tropical, Lower Austral, Upper Austral, Transition, Boreal, and Arctic (Plate III). These several divisions are termed climatic provinces, and are assumed to coincide with those of the life-regions as mapped by C. Hart Merriam. The basis for classification is mainly temperature. In the main, the northern boundaries of the provinces and their higher limits in mountainous regions are determined by the temperature of the season of growth and reproduction among plants and animals; while their southern boundaries or lower limits on the mountains are determined by the temperature of a brief period during the hottest portion of the year. A more definite account of the reasons for choosing these limitations will be given later in discussing life areas. While the principal basis for establishing climatic provinces is temperature, many other conditions are also recognised, chief among which is precipitation. Several of the climatic provinces have two divisions, namely, a humid and an arid, the dividing line being
approximately the one hundredth meridian. These are well defined and important in the tropical, austral, and transitional, but less definite and less well known in the boreal and arctic provinces. The regions embraced in the several climatic provinces named above, as well as their humid and arid divisions, so far as now understood, are shown on the accompanying map.
The Tropical Province (Plate III).—This is the most southern of the climatic provinces which it is convenient to recognise in North America, and includes the West Indies, Central America, and southern Mexico, together with a narrow strip on each coast of northern Mexico and the extreme southern ends of the peninsulas of Florida and of Lower California. While the land areas in this widely extended province present conspicuous differences, their climate in general is characterized by a high mean annual temperature with but moderate seasonal or daily variations, and by the occurrence in general of a wet and a dry season each year. The prevailing winds are the northeast trades. While the average yearly temperature is high, being in general about 80° F., the heat in summer is less intense than in many portions of the austral provinces. In winter the temperature does not fall sufficiently to produce frost, except on the higher mountains, which, in fact, belong to one or more of the other provinces named above. On ascending the mountains a rapid change to cooler and even to frigid zones is experienced. Snow occurs on the higher portions of the mountains of Central America and Mexico, and in a few instances, as on the great volcanic cones in sight from the City of Mexico, is perennial. On the mountains just referred to all of the several climatic provinces are represented by well-characterized zones, arranged one above another, and presenting in epitome the general changes one would experience in travelling from the Gulf of Mexico to the Arctic Ocean.
In marked contrast to the prevailing uniformity of temperature at corresponding elevations throughout the tropical provinces is the inequality in rainfall in reference to both seasonal periods and differences in geographical position.